1. Field of the Invention
The present invention relates to a piezoelectric device which is operable in the thickness shear vibratory mode and a manufacturing method therefor, more particularly, to a piezoelectric device such as an energy-trapped-type piezoelectric device, a piezoelectric filter or, a piezoelectric discriminator, and a manufacturing method therefor.
2. Description of Related Art
A conventional piezoelectric device which is operable in the thickness shear vibratory mode has been manufactured as shown in FIGS. 1a to 1j. The manufacturing method therefor will be described below with reference to FIGS. 1a to 1j.
First of all, a pair of electrodes 27 of Cu for a polarization process are formed on both of the top and bottom surfaces of a piezoelectric mother plate 2 having a shape a rectangular parallelepiped shown in FIG. 1a, and then, a predetermined direct-current voltage is applied between the pair of electrodes 27 so as to polarize the piezoelectric mother plate 2 in a direction of the thickness thereof as shown in FIG. 1b. Thereafter, the pair of electrodes 27 are removed from the piezoelectric mother plate 2 as shown in FIG. 1c, and then, the piezoelectric mother plate 2 is sliced so as to make plural sliced plates 9, as shown in FIGS. 1d and 1e.
Thereafter, plural pairs of circular electrodes 3a and 3b for a vibration and plural pairs of rectangular terminal electrodes 5a and 5b are formed on a top surface of the sliced plate 9 so that each pair of electrodes 3a and 3b are opposed to each other, each pair of electrodes 5a and 5b are opposed to each other, each electrode 3a is electrically connected to a corresponding electrode 5a, and each electrode 3b is electrically connected to a corresponding electrode 5b, as shown in FIG. 1f. Further, plural sets of circular electrodes 4a and 4b for the vibration and rectangular terminal electrode 5c are formed on a bottom surface of the sliced plate 9 so that the electrodes 4b and 4b respectively oppose to the electrodes 3a and 3b, the electrode 5c is located between the electrodes 5a and 5b, and each set of electrodes 4b, 4b and 5c are electrically connected to each other, as shown in FIG. 1i.
Thereafter, as shown in FIG. 1g, the sliced plate 9 is cut along lines c--c' so as to make plural piezoelectric vibration elements 30, each element 30 comprising a piezoelectric plate 1 of a rectangular shape having a long side and a short side, as shown in FIG. 1h. The direction of the polarization axis of each piezoelectric vibration element 30 as indicated by a polarization vector P is parallel to the long side of the piezoelectric plate 1. Thereafter, leads 11a, 11b and 11c are soldered on the electrodes 5a, 5b and 5c of the piezoelectric vibration element 30, respectively, as shown in FIG. 1i. Finally, the piezoelectric vibration element 30 is fully packaged with a resin outer package 13 of epoxy resin so as to make closed cavities 12 having a predetermined volume on the electrodes 3a, 3b, 4b and 4b and in the vicinity thereof, resulting in a conventional piezoelectric device, as shown in FIG. 1j.
In the conventional piezoelectric device manufactured as described above, the direction of the polarization axis of each piezoelectric vibration element 30 as indicated by the polarization vector P is parallel to the long side of the piezoelectric plate 1, as shown in FIG. 2. It is to be noted that no electrodes are drawn in FIG. 2. In FIG. 2, l.sub.a denotes a length in the direction of the short side of the piezoelectric plate 1, and l.sub.b denotes a length of in the direction of the long side thereof, wherein l.sub.a &lt;l.sub.b.
However, in the conventional piezoelectric device packaged with the resin outer package 13 as described above, there may occur a thermal stress between the resin outer package 13 and the piezoelectric plate 1 because of the expansion or the contraction thereof due to a difference between the thermal expansion coefficients of the resin outer package 13 and the piezoelectric plate 1 and a change in the environmental temperature. If the thermal stress to be applied to the piezoelectric plate 1 changes, the resonance frequency of the piezoelectric device may change. Namely, the resonance frequency of the piezoelectric device may be shifted by a change in the temperature thereof.
FIG. 3 shows a deviation .DELTA.f.sub.0 in the resonance frequency characteristic on the environmental temperature of the conventional piezoelectric device having the polarization direction parallel to the long side of the piezoelectric plate 1 as shown in FIG. 2, wherein the abscissa represents the environmental temperature, and the ordinate represents the deviation .DELTA.f.sub.0 in the resonance frequency f.sub.0 (T) thereof with a reference of a resonance frequency f.sub.0 (20) in an environmental temperature of 20 .degree. C., which is represented by the following equation (1). EQU .DELTA.f.sub.0 =f.sub.0 (T)-f.sub.0 (20) (1)
As is apparent from FIG. 3, the resonance H frequency f.sub.0 of the conventional piezoelectric device changes depending on a change in the environmental temperature, and also, the conventional piezoelectric device has a negative temperature characteristic with respect to the resonance frequency f.sub.0 thereof since the resonance frequency f.sub.0 decreases as the environmental temperature increases.
Further, a piezoelectric device having the direction of the polarization axis parallel to the short side of the piezoelectric plate 1 can be made by the conventional manufacturing method, as shown in FIG. 4.
FIG. 5 shows a deviation .DELTA.f.sub.0 in the resonance frequency characteristic depending on the environmental temperature of the conventional piezoelectric device having the polarization direction parallel to the short side of the piezoelectric plate 1 as shown in FIG. 4.
As is apparent from FIG. 5, the resonance frequency f.sub.0 thereof is shifted depending on a change in the environmental temperature, however, the piezoelectric device shown in FIG. 4 has a positive temperature characteristic with respect to the resonance frequency f.sub.0 thereof since the resonance frequency f.sub.0 increases as the environmental temperature increases.
As described above, the problem exists that there is a relatively large change in the resonance frequency f.sub.0 due to a change in the environmental temperature in the piezoelectric devices shown in FIGS. 2 and 4 which are made by the conventional manufacturing method. Further, since there is a relatively large change in the resonance frequency f.sub.0 thereof due to the environmental temperature, it is necessary to measure the resonance frequency f.sub.0 of a piezoelectric device keeping the environmental temperature constant, when checking whether or not each of the manufactured piezoelectric devices is an off-specification product, resulting in a relatively high cost for detecting off-specifications products.